Estimating cliff retreat in southern California considering sea level rise using a sand balance approach

A sand balance coastal profile model for estimating cliff and shoreline retreats considering sea level rise is discussed. The model, specifically designed for cliffed coasts fronted by sandy beaches, conditionally permits beach and cliff retreat to occur independently, and includes subaerial cliff erosion and external beach sand sources. The model accommodates complex nearshore and inland topography, high volume beaches, and variable cliff composition, and is suitable where local sand balance is thought to be a primary controller of coastal evolution over decade-century time scales. Designating an upper active beach boundary and beach-cliff intersection divides the coastal profile into active beach and cliff sections separated by a back beach buffer. The buffer acts as a sand reservoir and delays marine driven cliff erosion, resulting in lower estimated cliff retreat compared to previous models neglecting protective beaches. The model was applied on 21 km of cliffs in Marine Corps Base Camp Pendleton, California considering sea level rise ranging from 0.5 to 2 m over 100 yrs using 207 profiles, sand budget deficits estimated from historical data, and sand inputs from terrestrial erosion estimated from a time series of lidar data. Modeled mean and maximum scenario cliff retreats ranged from 4-87 m and 21-179 m, respectively, and provide order of magnitude estimates, but are reliant on model assumptions and do not include potential coastal changes unrelated to local sand balance. The results underscore the influence of protective beaches on cliff retreat.